Broadly tunable single longitudinal mode output produced from multi-longitudinal mode seed source

ABSTRACT

An oscillator system is provided which produces a single mode output from a multimode seed beam and a multimode optical power source. The seed source employed in the oscillator system produces a multimode seed beam that is used to injection seed a multimode optical power source. The optical power source has a short cavity such that the longitudinal modes of the multimode optical power source are sufficiently spaced apart that a mode of the optical power source can be positioned relative to the multimode seed beam such that the beam does not significantly overlap with neighboring longitudinal modes of the optical power source. By positioning a single longitudinal mode to overlap with the seed beam a high energy single longitudinal mode output is produced. The oscillator system preferably includes an output controller loop which serves to center the longitudinal mode output of the optical power source relative to the center of the seed beam. The oscillator system also preferably includes a tuner controller which enables the oscillator system to scan over a series of wavelengths.

RELATIONSHIP TO COPENDING APPLICATIONS

This application is a International 371 of PCT/US95/07189, filed Jun. 6,1995 which is a continuation-in-part of application Ser. No. 08/305,032,filed Sep. 13, 1994, now U.S. Pat. No. 5,577,058.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an oscillator system such as a opticalparametric oscillator or a laser. More specifically, the presentinvention relates to a tunable oscillator system which produces a singlelongitudinal mode output.

2. Description of Related Art

Tunable oscillator systems with relatively high output energies andnarrow linewidths have a number of scientific and engineeringapplications. Narrow linewidth gas phase spectroscopy has becomeimportant in combustion research and atmospheric monitoring. Tunableoscillator systems which produce a single mode bandwidth over a broadtuning range are ideally suited for these applications.

Most tunable oscillator systems are implemented using lasers, such asdye lasers which are based on gain media dyes, each of which are tunableover a range of 15-100 nanometers. These systems are desirable becauseof the narrow linewidths and relatively high energy achieved by thesesystems. However, dye laser systems are cumbersome for applicationsrequiring a wide tunable range because the dye gain medium must bechanged as the output is tuned beyond the 15 to 100 nanometer range ofthe particular dye laser being used.

Optical parametric oscillators (OPO) and amplifiers represent anothertype of tunable oscillator system. OPO systems are tuned by rotating theangle of the optical gain medium relative to the optical path usingservo techniques well known in the art. A variety of optical parametricgain media can be employed in OPOs, such as β-barium borate (BBO),lithium tri-borate (LBO), cesium borate (CBO), and potassiumtitanyl-phosphate (KTP), each of which provide a different tunablerange. OPOs are particularly interesting because of the wide range oftunability which they provide. For instance, tunable ranges from lessthan 400 nanometers to over about 2500 nanometers have been achieved byOPOs using β-barium borate as the optical gain media.

High power single longitudinal mode outputs have been achieved byinjection seeding a single mode seed source into a high power opticalpower source. The single mode seed sources used to produce a high powersingle longitudinal mode output are generally too weak to be used bythemselves as a single mode output.

For example, Bosenberg, et al., (J. Opt. Soc. Am. B 10 1716-1722 (1993))employs a tunable single frequency parametric frequency-conversionsystem in which single frequency pump radiation is obtained from thesecond harmonic radiation of a 10-Hz commercial Nd:YAG laser that isinjection seeded, SLM and flash-lamp pumped. Then, by making the cavityof the seed source OPO very short (5 cm), Bosenberg, et al. produces anarrow OPO bandwidth of approximately 0.02 cm⁻¹ which is used as asingle longitudinal mode seed source. In order to tune the OPO system ofBosenberg, et al., it is necessary to carefully control the cavitylength of the seed source, in addition to the crystal angle and tuningmirror, in order to maintain the production of a single longitudinalmode output over a broad tuning range.

Komine, et al., (Lasers '90: Proceedings of the 13th InternationalConference on Lasers and Applications. San Diego, Calif., Dec. 10-14,1990, (STS Press, 1991) pp. 612-618) teaches an alternative approach forthe production of an OPO bandwidth of less than 0.1 cm⁻¹ for use as asingle longitudinal mode seed source through the use of an intra-cavityetalon which serves to filter out unwanted modes. Etalons employwavelength selective coatings which function as a spectral filter toisolate the desired single longitudinal mode output. Because thecoatings used in etalons are wavelength selective, multiple optics arerequired to cover the tunable range of the oscillator system. Etalonsalso require angle tuning which adds an undesirable level of complexityto the production of a single longitudinal mode output, furtherencumbering one's ability to employ the single longitudinal mode outputin a tunable oscillator system.

Spectral filters have also been used in the production of a singlelongitudinal mode output for use as a seed beam. For example, Fix, etal. (Digest on Conference of Lasers and Electrooptics '94 p.199-200)employs an extra-cavity etalon as a spectral filter in order to producea single mode OPO output having a bandwidth of approximately 0.03 cm⁻¹.As with the use of etalons, spectral filters, such as the grating usedby Fix, et al., add an undesirable level of complexity to the productionof a single longitudinal mode output which encumbers one's ability toemploy the single longitudinal mode output in a tunable oscillatorsystem.

The requirement that a single longitudinal mode seed source be used toproduce a high energy single longitudinal mode output is disadvantageousin view of the complexity associated with producing a singlelongitudinal mode seed source. As discussed above, limiting the cavitylength of the seed source as is taught by Bosenberg, et al. or usingintra-cavity etalons, extra-cavity etalons or gratings to filter out adesired single mode seed beam, as is taught by Komine, et al. and Fix,et al., adds an undesirable level of complexity to the oscillator systemwhich greatly limits the ability of these systems to scan over a broadrange of wavelengths. It is therefore an object of the present inventionto provide a tunable oscillator system which produces a high energysingle longitudinal mode output from a multimode seed beam.

SUMMARY OF THE INVENTION

The present invention relates to an oscillator system which produces asingle mode output from a multimode seed beam and a multimode opticalpower source. As referred to herein, an oscillator system refers to anyoscillator system which produces an optical output and includes, forexample, optical parametric oscillator systems and laser systems. Theseed source employed in the oscillator system produces a multimode seedbeam that is used to injection seed a multimode optical power source.According to the present invention, the optical power source has a shortcavity such that the longitudinal modes of the multimode optical powersource are sufficiently spaced apart that a mode of the optical powersource can be positioned relative to the multimode seed beam such thatthe seed beam does not significantly overlap with neighboringlongitudinal modes of the optical power source. By positioning a singlelongitudinal mode to overlap with the seed beam where the seed beam doesnot significantly overlap with neighboring longitudinal modes of theoptical power source, a high energy single longitudinal mode output isproduced.

The multimode seed beam preferably has a bandwidth, defined as the fullwidth half max of the spectral output, of between about 0.025 cm⁻¹ and2.5 cm⁻¹. The seed beam most preferably has a bandwidth of between about0.05 cm⁻¹ and 0.20 cm⁻¹.

The optical power source should have an optical cavity length that issufficiently short to cause the longitudinal modes of the multimodeoptical power source to be sufficiently spaced apart so that themultimode seed beam does not overlap with neighboring longitudinal modesof the optical power source. Since the spacing of the longitudinal modesequals c/2L where c is the speed of light and L is the optical length ofthe cavity, the optical power source preferably has an optical cavitylength less than or equal to about c/2bw where c is the speed of lightand bw is the bandwidth of the multimode seed beam. It is preferred thatthe optical power source have an optical cavity length of between about0.2 cm and 20 cm.

The oscillator system preferably includes an output controller loopwhich serves to center the longitudinal mode output of the optical powersource relative to the center of the seed beam.

The oscillator system of the present invention preferably includes atuner controller which enables the oscillator system to scan over aseries of wavelengths.

Other aspects and advantages of the present invention can be seen uponreview of the figures, the detailed description, and the claims whichfollow.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 provides an overview block diagram of the tunable oscillatorsystem of the present invention.

FIGS. 2A-C illustrate the positioning of the multimode seed beamrelative to the longitudinal modes of the optical power source whereinFIG. 2A illustrates the situation where the longitudinal mode spacing istoo narrow to enable the seed beam to be positioned such that it onlyoverlap with one longitudinal mode of the optical power source, FIG. 2Billustrates the misalignment of the seed beam relative to onelongitudinal mode of the optical power source such that it overlaps withmore than one longitudinal mode, and FIG. 2C illustrates a multimodeseed beam centered upon one of the longitudinal modes of the opticalpower source where the longitudinal modes are sufficiently spaced apartthat the multimode seed beam does not substantially overlap withneighboring longitudinal modes of the optical power source.

FIG. 3 illustrates a preferred embodiment of the seed source employed inthe oscillator system of the present invention.

FIG. 4 illustrates a preferred embodiment of the optical power sourceemployed in the oscillator system of the present invention.

FIG. 5 illustrates an embodiment of the oscillator system of the presentinvention including an output controller loop for monitoring thefrequency of the output beam relative to the seed beam in which agrating is employed as the frequency separating device.

FIG. 6 illustrates the seed source employed with the output controllerloop illustrated in FIG. 5.

FIG. 7 illustrates an embodiment of the oscillator system of the presentinvention which includes an output controller loop for monitoring thefrequency of the output beam relative to the seed beam in which aninterferometer is employed as the frequency separating device.

FIG. 8 illustrates the detection system as including a first splitdetector upon which a fringe pattern from the seed beam is projected anda second spit detector upon which a fringe pattern from the output beamis projected.

FIGS. 9A-B illustrate a scanning Fabry-Perot Interferometer in whichFIG. 9A provides a top view of the interferometer and FIG. 9B provides aside view of the interferometer.

DETAILED DESCRIPTION

The present invention relates to an oscillator system which produces asingle mode output from a multimode seed beam and a multimode opticalpower source. As referred to herein, an oscillator system refers to anyoscillator system which produces an optical output and includes, forexample, optical parametric oscillator systems and laser systems. Asingle mode and multimode output refers to a single and multiplelongitudinal mode output respectively. The optical power source isreferred to as a multimode optical power source because multiplelongitudinal modes are generated by the optical power source in theabsence of injection seeding. The optical power source has a cavitywhich has an optical path length. In general, optical path length refersto a physical distance multiplied by the refractive index of thematerial occupying that distance. Hence, the optical length of thecavity equals the sum of the distances making up the cavity multipliedby the refractive index of each distance. The optical cavity length, asdiscussed herein, is generally directed to optical power sources withlinear cavities which have an optical cavity length L. However, itshould be understood that the discussions regarding the optical cavitylength of a linear optical power source can be readily adapted to ringoptical power sources by substituting L_(R) for 2L where L_(R) is theoptical cavity length of a ring optical power source.

According to the present invention, the optical power source has a shortcavity such that the longitudinal modes of the multimode optical powersource are sufficiently spaced apart that a mode of the optical powersource can be positioned relative to the multimode seed beam such thatthe seed beam does not significantly overlap with neighboringlongitudinal modes of the optical power source. By positioning a singlelongitudinal mode of the optical power source to overlap with the seedbeam where the seed beam does not significantly overlap with neighboringlongitudinal modes of the optical power source, a high energy, singlelongitudinal mode output is produced.

FIG. 1 provides an overview block diagram of the tunable oscillatorsystem of the present invention. The oscillator system includes amultimode seed source 10 which generates a multimode seed beam on line11. The seed beam is used to injection seed a multimode optical powersource 12 which generates a single longitudinal mode output beam 13. Theoscillator system also includes a tuner controller 19 connected to theseed source 10 and the optical power source 12 across lines 20 and 21which cooperatively tunes the seed source 10 and the optical powersource 12 to generate an output beam 13 at a selected wavelength.Incorporation of the tuner controller into the oscillator system enablesthe oscillator system to be tuned over a series of wavelengths.

The tunable oscillator system also includes pump sources 14 and 15 whichdrive the multimode seed source 10 and the multimode optical powersource 12 respectively. Optionally, pump sources 14 and 15 may be asingle pump source. When the multimode seed source 10 or the multimodeoptical power source 12 is an optical parametric oscillator, it ispreferred that a single longitudinal mode pump source be used. When themultimode seed source 10 or the multimode optical power source 12 is alaser, either a single or a multimode longitudinal mode pump source maybe used. However, when a laser is used, the pump source is preferably amultimode longitudinal mode pump source.

In the oscillator system of the present invention, a single mode outputis produced from a multimode optical power source 12 by injectionseeding the multimode optical power source 12 with a multimode seed beam11. The optical cavity length of the optical power source 12 is madesufficiently short to cause the longitudinal modes of the multimodeoptical power source 12 to be sufficiently spaced apart so that themultimode seed beam 11 does not substantially overlap with neighboringlongitudinal modes of the optical power source 12. The seed beam 11 ispreferably centered about a particular longitudinal mode of the opticalpower source. As illustrated in FIGS. 2A-C, when the spacing of thelongitudinal modes of the optical power source are too narrow to enablethe seed beam to be positioned such that it only overlaps with onelongitudinal mode of the optical power source (FIG. 2A) or when the seedbeam is misaligned relative to one longitudinal mode of the opticalpower source such that it overlaps with more than one longitudinal mode,(FIG. 2B), a multimode output is produced. However, if the multimodeseed beam is approximately centered upon one of the longitudinal modesof the optical power source and the longitudinal modes are sufficientlyspaced apart such that the multimode seed beam does not substantiallyoverlap with neighboring longitudinal modes of the optical power source,a single longitudinal mode output is produced (FIG. 2C). As illustratedin FIG. 2C, it is preferred that the optical power source be adjusted toapproximately center the seed beam about one of the longitudinal modes,thereby causing the frequency of the seed beam to approximately equalthe frequency of the output beam.

It is believed that the ability of the oscillator system of the presentinvention to convert a multimode optical power source to a single modeoutput through injection seeding using a multimode seed beam is relatedto the physical phenomenon known as injection seeding. Injection seedingrefers to the phenomenon by which a single mode seed beam is used toproduce a single mode output from an otherwise multimode oscillator.Injection seeding is believed to work by introducing a single mode seedbeam which, if the seed beam has sufficient power, enhances that singlemode of the optical power source over other modes of the optical powersource. Multiple modes are not produced by the optical power sourcebecause the single mode seed beam, having only a single mode, is onlyable to enhance a single mode of the multimode optical power source.

According to the present invention, the individual longitudinal modes ofthe optical power source are caused to be sufficiently spaced apart sothat the multimode seed beam only significantly overlaps with a singlemode of the optical power source. By causing the individual longitudinalmodes of the optical power source to be sufficiently spaced apart suchthat the multimodes of the seed beam only overlap with a single mode ofthe multimode optical power source input, only a single mode of themultimode optical power source input is enhanced by the seed beam,thereby causing the otherwise multimode optical power source to producea single mode output.

The longitudinal mode spacing of a linear optical power source is equalto c/2L where c is the speed of light and L is the optical cavity lengthof the optical power source. The longitudinal mode spacing of ringoptical power sources is c/L_(R) where L_(R) is the round trip opticalcavity length of the ring. By making the optical cavity length of theoptical power source very short, the individual longitudinal modes ofthe optical power source can be spaced apart. According to the presentinvention, the optical cavity length of the optical power source is madesufficiently short such that the longitudinal mode spacing of theoptical power source is greater than the bandwidth of the seed beam. Asa result, it is possible to position the modes of the optical powersource such that only one of the modes of the optical power sourceoverlaps with the multimode seed beam. As a result, only one mode of themultimode optical power source is enhanced.

The bandwidth of the multimode seed beam, defined as the full width halfmaximum of the spectral output, is preferably between about 0.025 cm⁻¹and 2.5 cm⁻¹. The multimode seed beam most preferably has a bandwidth ofbetween about 0.05 cm⁻¹ and 0.2 cm⁻¹. Given that the longitudinal modespacing is c/2L and the mode spacing should be equal to or greater thanabout the bandwidth of the multimode seed beam, the optical cavitylength of the optical power source should have a length equal to or lessthan c/2bw where c is the speed of light and bw is the bandwidth of theseed beam. The optical cavity length of the optical power source ispreferably between about 0.2 cm and 20 cm, most preferably about 2 cm.

When the single longitudinal mode to be enhanced is not centeredrelative to the multimode seed beam, the frequency of the single modeoutput will be different than the frequency of the seed beam. Whetherthe seed beam is centered on one of the longitudinal modes may beevaluated by comparing the frequency of the output beam to the frequencyof the seed beam. A shift in the frequency of the output beam to the redindicates that the cavity should be shortened while a shift in thefrequency of the output beam to the blue indicates that the cavityshould be lengthened. In order to align the single mode output of theoptical power source with the multimode seed beam, the optical cavitylength of the optical power source may be adjusted. Alternatively, thefrequency of the seed beam may be adjusted. An output controller loopfor controlling the frequency of the output beam relative to the seedbeam is discussed herein with regard to FIG. 5.

With regard to the seed source 10, any oscillator which produces anarrow line output that has a bandwidth less than the longitudinal modespacing of the optical power source may be used. Examples of suitableoscillators include, but are not limited to, OPOs, dye lasers, diodelasers, Ti:sapphire lasers and neodymium doped lasers.

FIG. 3 illustrates a narrow line OPO as a preferred seed source 10 thatmay be employed in the tunable oscillator system of the presentinvention. The narrow line OPO consists of a resonant oscillator OPOcavity defined by high reflector HR₁ 30, diffraction grating 31, andhigh reflector HR₃ 32. This resonant oscillator OPO cavity defines anoptical path 33 through a gain material 34. The gain material may be anynon-centrosymmetric crystal having a non-linear polarizability, wideoptical transmission, and a large non-linear coefficient. Examples ofsuitable gain materials include, but are not limited to β-barium borate,lithium tri-borate, cesium borate, and potassium titanyl-phosphate. Thegain material 34 is mounted on an angle tunable mount 35, usingtechniques well known in the art. The angle of the gain material may beadjusted relative to the optical path either manually or with the aid ofa microprocessor using techniques known in the art.

In order to obtain higher gain, it is possible to use twocounter-rotating gain media as described in U.S. Pat. No. 5,047,668 toBosenberg. The use of two counter-rotating gain media serves to maintainthe alignment of the narrow line OPO and to minimize walk-off by thenarrow line OPO. One advantage provided by increasing the gain is thatit enables the angle of incidence on the grating to be increased which,in turn, enables narrower line widths to be achieved.

The narrow line OPO is coupled to a tuning servo 36 which tunes theresonant oscillator OPO cavity by rotating the gain material 34 toadjust the phase match angle within the gain material 34, and to rotatethe mirror HR₃ 32 to keep reflection onto the grating at the correctangle to match changes in wavelength. Thus, the tuning servo is coupledto the angle tunable mount 35 across line 47, and coupled to acontrollable mount 37 for the high reflector HR₃ 32 across line 38. Thetuning servo 36 is also coupled across line 39 to the optical powersource 12.

The narrow line OPO illustrated in FIG. 3 includes structure forlongitudinally pumping the BBO material 34. This structure includes adichroic mirror 41 and a dichroic mirror 42. The dichroic mirrors 41 and42 are preferably transmissive from about 400 nm to 2500 nm. A pump beamis supplied along path 43 into mirror 41 and reflected through the gainmaterial along path 44 to mirror 42 where it is directed out of thecavity. The high reflectors HR₁ 30 and HR₃ 32 are broad band visiblehigh reflectors.

The dichroic high reflector 41 is reflective at the pump wavelength at45 degrees and transmits through the tunable range of the gain material34. High reflector 42 is reflective at the pump beam wavelength at 45degrees to direct the pump beam out of the resonant path after a singlepass through the gain medium and is transmissive over the tunable rangeof the gain material 34.

The grating 31 preferably have 2700 lines (or grooves) per millimeter,optimized for a tuning range of 400 to 700 nm in a single diffractionorder. The grating 31 is a wavelength selective element which serves tocontrol the linewidth of the seed beam 40 to less than one wave number(1 cm⁻¹), and preferably less than 0.2 cm³¹ 1. The grating 31 may becontrolled either manually or with the aid of a microprocessor usingtechniques known in the art.

With regard to optical power source, any oscillator may be used as theoptical power source in the oscillator system of the present inventionprovided that the optical cavity length can be made sufficiently shortsuch that the longitudinal mode spacing of the optical power source isapproximately equal to or greater than the bandwidth of the seed beam.

A preferred embodiment of the multimode optical power source employed inthe oscillator system of the present invention is illustrated in FIG. 4.The optical power source illustrated in FIG. 4 includes an OPOrepresented by components 110 through 120 and an optical parametricamplifier indicated by components 121 through 127 which may optionallybe included.

The optical power source illustrated in FIG. 4 includes a gain medium110 mounted within a resonator cavity 120 defined by a high reflector111 and an output coupler 112 where the optical distance between thehigh reflector 111 and output coupler 112 defines the optical cavitylength (L). The output coupler 112 has a reflecting surface facing thehigh reflector 111. The gain media is preferably an optical parametricgain media. Examples of optical parametric gain media include, but arenot limited to β-barium borate, lithium tri-borate, cesium borate, andpotassium titanyl-phosphate. It is preferred that p-barium borate beemployed as the gain medium.

The multimode optical power source illustrated in FIG. 4 furtherincludes a first input dichroic mirror 113 and a first output dichroicmirror 114 positioned outside the resonant cavity for directing a firstpump beam through the parametric gain medium 110. Thus, a pump beam issupplied along path 115 into dichroic mirror 113 which is mounted at -45degrees to the optical path 116 of the resonator. The pump beam 115 isthen directed through the parametric gain medium 110 to dichroic mirror114 mounted at +45 degrees to the optical path 116. At dichroic mirror114, the remaining pump beam is reflected along path 117 to a beamblocking mechanism, schematically shown at 118.

As illustrated in FIG. 4, the high reflector 111 is mounted on anadjustable base 119 so that fine adjustments of the optical cavitylength can be made. As discussed herein with regard to FIG. 5, theadjustable base may be controlled by an output controller which adjuststhe alignment of the one of the longitudinal modes of the optical powersource with the seed beam.

In instances where the output generated by an OPO type optical powersource is not sufficient, the OPO may be modified to include an unstableresonator coupled with increased pump energy. The unstable resonatorprovides greater spacial mode selectivity, thereby making it possible toincrease the pump energy and, consequently, increase the output energy.An OPO modified to include an unstable resonator is described in U.S.patent application Ser. No. 08/111,083 entitled MASTER OPTICALPARAMETRIC OSCILLATOR/POWER OPTICAL PARAMETRIC OSCILLATOR, filed Aug.24, 1993, which is incorporated herein by reference. If an unstableresonator is used, it is preferred to divide the gain into twocounter-rotating gain media as described in U.S. Pat. No. 5,047,668 toBosenberg which is incorporated herein by reference. The twocounter-rotating gain media act to preserve the alignment of the opticalpower source as it is tuned. Alternatively, an unstable resonator may beused which employs a single gain media in combination with one curvedmirror and one flat mirror. In this embodiment, if the pumped light isincident on the gain media from the direction of the curved mirror, thealignment of the optical power source is preserved as it is tuned.

Optionally, the optical power source illustrated in FIG. 4 can furtherinclude an optical parametric amplifier (OPA) indicated by components121 through 127. Specifically, the OPA includes a second input dichroicmirror 121 and a second output dichroic mirror 122 positioned outsidethe resonant cavity for directing a first pump beam through secondparametric gain medium 123. Thus, a pump beam is supplied along path 124into dichroic mirror 122 which is mounted at -45 degrees to the opticalpath 125. The pump beam 124 is then directed through the parametric gainmedium 123 to dichroic mirror 122 mounted at +45 degrees to optical path125. At dichroic mirror 122, the remaining pump beam is reflected alongpath 126 to a second beam blocking mechanism, schematically shown at127.

In a preferred embodiment, the tunable oscillator system furtherincludes an output controller loop which monitors the output of the seedsource and optical power source in order to adjust the frequency of oneof the longitudinal modes of the optical power source relative to theseed beam. Using an output control loop in a tunable oscillator system,one is able to center the output of the oscillator system on the seedbeam frequency, thereby providing cleaner seeding, better stability andminimizing mode hops due to the frequency of the output beam not beingtuned relative to the seed source.

Inclusion of a control loop provides these advantages to tunableoscillator systems regardless of whether the seed source used in thetunable oscillator system is a single mode or a multiple mode seed beam.Thus, a further aspect of the present invention is the use of a controlloop, as described herein, in combination with either single mode or amultiple mode seed beam.

The output controller loop includes a frequency separating device whichreceives a portion of the output beam and a portion of the seed beam.The frequency separating device displaces the portions of the outputbeam and seed beam to different positions based on their frequencies.The output controller loop also includes a detection system whichreceives light from the frequency separating device, detects theposition of the light and generates a signal corresponding to theposition of the light. The output controller loop also includes anoutput controller which receives the signal from the detection systemand produces a control signal in response for modifying the frequency ofthe optical power source. In one embodiment of the output controllerloop, the frequency separating device includes a diffraction grating offof which the output beam and seed beam are diffracted where the angle ofdiffraction off of the diffraction grating changes based on frequency.The light diffracted from the diffraction grating is then conveyed to adetection system which compares the frequency of the output beam to thefrequency of the seed beam based on the angle of diffraction of thelight. This embodiment is illustrated with regard to FIGS. 5 and 6.

FIG. 5 illustrates the output controller loop as including a beamsplitter 25 which conveys a portion of the output beam 13 by opticalpath 27 to seed source 10. The seed source illustrated in FIG. 6 isconsistent with the seed source illustrated in FIG. 3 with the followingadditions. The output beam conveyed by optical path 27 enters the seedsource and contacts mirror 46 which then conveys the output beam tograting 31 along a path parallel to path 33. The output beam is thenreflected off mirror 32, back to grating 31 and returned to mirror 46 inwhich the output beam exits by optical path 29. The output beam conveyedon optical path 29 is then directed to split detector 24 as shown inFIG. 5. If the frequency of the output beam is not the same as thefrequency of the seed beam, the output beam will be physically displacedby its diffraction from grating 31. The angular displacement of theoutput beam after being reflected off of grating 31, either positive ornegative, indicates whether the frequency of the output beam is greaterthan or less than the frequency of the seed beam. If the frequency ofthe output beam equals that of the seed beam, the output beam will notbecome displaced by diffraction from the grating.

As illustrated in FIG. 5, the split detector 24 monitors whether theoutput beam, returned by optical path 29 has been positively ornegatively angularly displaced. In response, the split detector sends asignal to controller 15 which then sends a control signal by path 16 tooptical power source 12 to alter the optical cavity length of theoptical power source, thereby adjusting the frequency of the output beamto match the seed beam. Alternatively, the output controller can beattached to the tuner controller 19 to modify the frequency of eitherthe seed beam or the optical power source.

FIG. 7 provides an alternative embodiment for the output controller loopillustrated in FIG. 5. As illustrated in FIG. 7, the frequencyseparating device includes an interferometer 52 through which the outputbeam 27 and seed beam 56 are passed. The use of an interferometer as thefrequency separating device is preferred over diffraction gratings inview of the greater spectral resolution interferometers provide.

As illustrated in FIG. 8, when light from the output beam 27 and seedbeam 56 is passed through the interferometer 52, a seed beam fringepattern 58 and an output beam fringe pattern 60 are produced where thepositions of the fringes correspond to the frequency of the lightgenerating the fringe pattern. In a preferred embodiment, theinterferometer 52 is a Fabry-Perot Interferometer, more preferably ascanning Fabry-Perot Interferometer.

As illustrated in FIG. 7, the fringe patterns 58 and 60 produced bypassing the seed beam 56 and output beam 27 through the interferometerare conveyed to a detection system 62 including a first split detector64 upon which the fringe pattern 58 from the seed beam 56 is projectedand a second spit detector 66 upon which the fringe pattern 60 from theoutput beam 27 is projected. As illustrated in FIG. 8, the first splitdetector 64 is positioned in the path of one of the fringes of the seedbeam fringe pattern 58 while the second split detector 66 is positionedin the path of one of the fringes of the output beam fringe pattern 60.

As illustrated in FIG. 7, the first and second split detectors 64, 66each produce a signal indicating the position of the fringe patterns 58and 60 for the seed beam 56 and output beam 27 relative to the first andsecond split detectors. The signal produced by the first split detector64 is conveyed to a first output controller 65 which produces a controlsignal used to scan the interferometer and keep one of the fringes ofthe seed beam fringe pattern 58 projected on the first split detector64.

The signal produced by the second split detector 66 is used to evaluatewhether the frequency of the output beam is tuned relative to thefrequency of the seed beam. Specifically, the second split detector 66is positioned such that one of the fringes generated by the seed beamand the output beam will be centered on the first and second splitdetectors 64, 66 respectively when the frequency of the output beamequals the frequency of the seed beam. The signal produced by the secondsplit detector 66 is conveyed to a second output controller 67 whichsends a control signal by path 16 to optical power source 12 to adjustthe frequency of the output beam to match the seed beam.

As illustrated in FIG. 8, the output controller loop 62 may furtherinclude a third detector 68 positioned on a dark fringe, preferably nearthe center of the output beam fringe pattern. By comparing the signalfrom the large detector 68 to the total signal from the second splitdetector 66, one is able to determine the amount of energy that is notin the single mode. The information provided by the large detector 66regarding the amount of energy that is not in a single mode may be usedduring the initial alignment of the optical power source and to evaluateindividual data samplings.

It is preferred that the interferometer 52 used be a Fabry-PerotInterferometer, most preferably a scanning Fabry-Perot Interferometer.FIGS. 9A-B illustrate a scanning Fabry-Perot Interferometer as avariable air space Fabry-Perot Interferometer which includes apiezoelectric transducer 70 for controlling the spacing of the scanningFabry-Perot Interferometer. A top and side view of the scanningFabry-Perot Interferometer is provided in FIGS. 9A and 9B respectively.Forms of scanning Fabry-Perot Interferometers that may be used include,for example, a 410 Interferometer from Spectra Physics Components andAccessories Group. The Fabry-Perot Interferometer may optionally furtherinclude a strong negative cylindrical lens 72 positioned in front of theinterferometer 52 and a weak positive cylindrical lens 74 positionedbehind the interferometer 52, the pair of lenses serving to producestraight mode lines as opposed to circular mode lines as the output.

In an alternative embodiment, a detector array which can detect lightover a series of positions is employed as the detection system. Anexample of a detector array that may be used in the present invention isa detector array formed using a Hamamatsu Corp. Series S2301 lineararray and a Hamamatsu Corp. Series C2325 amplifier driver circuit. Adetector array is preferred in instances where a fixed interferometer isemployed. Fixed interferometers cannot be adjusted to align the fringepattern with the detection system. By using a detector array which candetect light over a series of positions, the need to align the fringepattern relative to the detection system is avoided.

Alternative methods for monitoring and matching the frequency of theoutput beam to the frequency of the seed beam can be envisioned and areintended to fall within the scope of the present invention. Anoscillator system of the present invention was constructed employing anamplified optical power source as described with regard to FIG. 4.Specifically, the gain medium 110 employed was β-barium borate (13 mm,cut at 28°, Type I, AR coated). High reflector 111 had 95% reflectivity.The output coupler 112 had 20% partial reflectivity. The optical cavitylength was set to about 2 cm. The pump energy employed was 8 mJ ofsingle frequency energy at 355 nm. The seed beam employed was amultimode beam having a bandwidth of 0.2 cm⁻¹. The output beam producedwas 2 mJ having a bandwidth of 0.023 cm⁻¹.

The output was then amplified by passing the output through an opticalparametric amplifier where the gain medium was β-barium borate (13 mm,cut at 28°, Type I, AR coated). Using 220 mJ of single frequency pumpenergy at 355 nm, the 2 mJ single mode output was amplified to 35 mJ,corresponding to the sum of the power of the signal and idler outputs.

The tunable oscillator system of the present invention obviates the needto use a single mode seed beam to injection seed an optical power sourcein order to produce a single mode output and, as such, is greatlysimplified over prior art tunable oscillator systems. Prior artoscillator systems require a single longitudinal mode seed beam which isgenerated either by shortening and carefully controlling the cavity ofthe seed source or by employing a spectral filtering device such as anintra-cavity etalon, an extra-cavity etalon or a grating. All of theseapproaches for producing a single longitudinal mode seed beam add anunnecessary level of complexity to the oscillator system which impedesthe ability of the oscillator system to scan over a range ofwavelengths.

By shortening the optical power oscillator in relation to the seed beamaccording to the present invention, a multimode seed beam may be used toproduce a single longitudinal mode output. Because a multimode seed beamis employed in place of a single mode seed beam, the oscillator systemof the present invention can scan over a greater number of wavenumbersand at a higher rate. The oscillator system of the present inventionshould enable more than 100 wavenumbers to be scanned and at a rate inexcess of 10 wavenumbers per hour.

The output controller loop of the present invention enables theautomated tuning of a seed source relative to an optical power source ina tunable oscillator system in order to produce a single longitudinalmode output. According to the present invention, the output controllerloop may be used in combination with a single mode seed source as wellas with a multimode seed source.

The foregoing description of preferred embodiments of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Obviously, many modifications and variations will be apparentto practitioners skilled in this art. It is intended that the scope ofthe invention be defined by the following claims and their equivalents.

What is claimed is:
 1. A tunable oscillator system comprising:a seedsource capable of producing a multimode seed beam; an optical powersource coupled to the seed source and injection seeded by the multimodeseed beam to generate an output beam, the optical power source having acavity with an optical length which causes a single mode output to beproduced by the optical power source; and a tuner controller coupled tothe seed source and the optical power source which controls a wavelengthof the output beam by controlling the seed source and the optical powersource.
 2. A tunable oscillator system according to claim 1 wherein themultimode seed beam has a bandwidth between about 0.025 cm⁻¹ and 2.5cm⁻¹.
 3. A tunable oscillator system according to claim 1 wherein themultimode seed beam has a bandwidth of between about 0.2 cm⁻¹ and 0.5cm⁻¹.
 4. A tunable oscillator system according to claim 1 whereinoptical power source is a linear optical power source.
 5. A tunableoscillator system according to claim 4 wherein the optical cavity lengthof the linear optical power source is less than or equal to about c/2bwwhere c is the speed of light and bw is the bandwidth of the multimodeseed beam.
 6. A tunable oscillator system according to claim 1 whereinoptical power source is a ring optical power source.
 7. A tunableoscillator system according to claim 6 wherein the optical cavity lengthof the ring optical power source is less than or equal to about c/bwwhere c is the speed of light and bw is the bandwidth of the multimodeseed beam.
 8. A tunable oscillator system according to claim 1 whereinthe optical cavity length of the optical power source is between about0.2 cm and 20 cm.
 9. A tunable oscillator system according to claim 8wherein the optical cavity length of the optical power source is about2.0 cm.
 10. A tunable oscillator system according to claim 1 wherein theseed source is an oscillator selected from the group consisting ofoptical parametric oscillators, dye lasers, diode lasers, Ti:sapphirelasers and neodymium doped lasers.
 11. A tunable oscillator systemaccording to claim 1 wherein the optical power source is an oscillatorselected from the group consisting of optical parametric oscillators,dye lasers, diode lasers, Ti:sapphire lasers and neodymium doped lasers.12. A tunable oscillator system according to claim 11 wherein theoptical power source is an optical parametric oscillator.
 13. Anoscillator system according to claim 12 wherein the optical power sourceis an amplified optical parametric oscillator.